This section provides background information related to the present disclosure which is not necessarily prior art.
Transfer gears are used in particular in motor vehicles with all-wheel drive in order to distribute an engine-generated torque, which is made available at a drive shaft of the transfer gear, to two output shafts of the transfer gear and thus to more than one driven axle. Here, each of the output shafts drives an axle of the motor vehicle or, if the motor vehicle has more than two driven axles, a final drive with drive-through facility.
Along the power flow from the engine to the driven axle, the transfer gear is normally arranged downstream of the main transmission of the drive engine, which serves for stepping up the torque generated by the drive engine.
Transfer gears are, depending on their type of construction, subdivided into so-called differential-controlled transfer gears and clutch-controlled transfer gears. In the case of differential-controlled transfer gears, a possibly manually or automatically lockable differential and/or a planetary gear set distributes the power flow to two output shafts that are permanently coupled to the drive shaft. In the case of clutch-controlled transfer gears, an automatically and/or manually actuated clutch, in particular a friction clutch, such as for example a multiplate clutch, or a viscous coupling with or a rigid coupling without rotational speed equalization, such as for example a claw coupling, distributes the power flow to two output shafts. Here, one output shaft, the so-called primary shaft, is permanently coupled to the drive shaft, and the remaining output shaft, the so-called secondary shaft, can be coupled to the drive shaft only as required.
Furthermore, transfer gears with activatable and deactivatable all-wheel drive are known which are duly not permanent but nevertheless have a differential gear and/or a planetary gear set which performs the distribution of the drive power when all-wheel drive is activated. In the case of this mixed embodiment, the all-wheel drive is differential-controlled but can also be deactivated and activated by way of one or more clutches. Since said transfer gears likewise have a clutch which activates and deactivates the secondary shaft, they are likewise regarded as clutch-controlled transfer gears.
For completeness, it is pointed out that transfer gears are also known which make do entirely without a clutch and without a differential gear or planetary gear set, for example if the wheels on the driven axles have freewheel-type hubs.
Clutch-controlled transfer gears are also referred to as “disconnect”-capable transfer gears, as they enable one of the two output shafts to be decoupled from the drive shaft, and thus for the through drive and thus the transmission of torque from the drive shaft to one of the two output shafts to be deactivated. Thus, clutch-controlled or “disconnect”-capable transfer gears have a drive shaft, an output shaft which is referred to as primary shaft and which is permanently coupled to the drive shaft, and an output shaft which is referred to as secondary shaft and which can be coupled to (and decoupled from) the drive shaft by way of a clutch.
Transfer gears may furthermore have an offset gear if the output shafts have an axial offset with respect to one another and/or one of the two output shafts has an axial offset with respect to a drive shaft which is or can be connected to the engine of a motor vehicle. As an offset gear, use is made of gear trains comprising a gearwheel set with intermeshing gearwheels which are for example in the form of spur gears and which are each arranged rotationally conjointly on the shafts which have an axial offset, and/or traction mechanism drives in which a torque is transmitted between the axially offset shafts by means of a traction mechanism which is looped around both shafts. In this case, use is preferably made of positively locking traction mechanism drives such as for example chain drives or toothed-belt drives, in which the torque is transmitted by way of wheels, which will hereinafter be referred to as sprockets and which are for example in the form of gearwheels and which have a corresponding positively locking profile, from a first shaft to the respective traction mechanism and from the traction mechanism to the second shaft, which has an axial offset with respect to the first shaft.
The traction mechanism and/or the gearwheel set of the offset gear is normally additionally used to convey oil from a low-oil sump to one or more bearings of the transfer gear which are situated at a higher level, in order for said bearings to be lubricated and cooled for the purposes of reducing the drive torque thereof and the temperature thereof during the operation of the motor vehicle.
A disadvantage of this is that, in the case of clutch-controlled or “disconnect”-capable transfer gears, said lubrication is absent when a secondary shaft, which is driven by way of the offset gear, is for example automatically or manually fully deactivated during the operation of the motor vehicle. Such complete immobilization of the secondary shaft is normally realized by way of one or more further separating clutches which are independent of the transfer gear clutch and which effect a decoupling of the secondary shaft from the gears which are driven by way of the secondary shaft and/or from the differential which is driven by way of the secondary shaft. This is realized for example when the all-wheel drive motor vehicle is being driven on a high-grip roadway with only one axle being driven by way of the primary shaft.
This leads to increased fuel and drive energy consumption owing to increasing friction and temperature of the now no longer lubricated bearings, and can lead to overheating, and in the worst case destruction, of the transfer gear, which in turn is associated with a considerable impairment of the roadworthiness of motor vehicles equipped therewith.
DE 39 42 411 A1 has disclosed a clutch-controlled or “disconnect”-capable transfer gear with an offset gear which comprises sprockets and chain and with a clutch in the form of a multiplate clutch. The transfer gear has a drive shaft, a primary shaft which is arranged coaxially with respect to the drive shaft and which is connected rotationally conjointly to the drive shaft, and a secondary shaft, which is arranged with an axial offset with respect to the drive shaft. The offset gear comprises a first sprocket, which is arranged coaxially with respect to the drive shaft and which is mounted so as to be rotatable relative to the drive shaft, a second sprocket, which is connected rotationally conjointly to the secondary shaft, and a chain which is looped around the two sprockets. The multiplate clutch is arranged coaxially with respect to the drive shaft and with respect to the primary shaft and has multiple first clutch discs, which are connected rotationally conjointly to the drive shaft and to the primary shaft arranged in a coaxial elongation of the drive shaft, and multiple second clutch discs, which are connected rotationally conjointly to the first sprocket and which are mounted in a housing of the transfer gear so as to be rotatable independently of the drive shaft.
Thus, the offset gear is decoupled from the drive shaft and from the primary shaft when the secondary shaft is deactivated by way of the multiplate clutch arranged coaxially with respect to the drive shaft and with respect to the primary shaft. A drive shaft bearing and a unilaterally encapsulated sprocket bearing are situated, in structural terms, in a closed-off space within the housing. The space is partially filled with oil, wherein the sprocket, which is connected to the secondary shaft arranged below the drive shaft, is situated entirely below the oil level. The transfer gear has a lubricant pump in order that drive shafts and sprocket bearings situated above the oil level are supplied with oil, and thus lubricated, even in the event of the secondary shaft being deactivated and, as a result, the chain being deactivated.
DE 10 2008 058 278 A1 has disclosed a clutch-controlled or “disconnect”-capable transfer gear with a housing, with an offset gear which comprises a gearwheel set, and with a clutch designed as a multiplate clutch. The transfer gear has, rotatably mounted in the housing, a primary shaft which is arranged coaxially with respect to the drive shaft, a clutch which is arranged coaxially with respect to said primary shaft and which is in the form of a multiplate clutch, and a secondary shaft which is arranged below the primary shaft with an axial offset and which is connected rotationally conjointly to a gearwheel of the gearwheel set. A lubricant pump conveys oil for the lubrication of drive shaft bearing, primary shaft bearing and multiplate clutch, and of gearwheel set bearings situated above the oil level, from a low-lying, partially oil-filled chamber, referred to as oil sump, within the housing of the transfer gear. To reduce splashing losses of the gearwheel set, an intermediate reservoir of which releases a part of the conveyed oil again with a delay is provided at mid-height between the primary shaft and oil sump. A drive shaft sealing ring which seals off the housing against oil losses is arranged with a spacing in the axial direction with respect to the drive shaft bearing as viewed along the drive or primary shaft.
The above-described clutch-controlled or “disconnect”-capable transfer gears which are known from the prior art and which have coaxially arranged drive shaft and primary shaft, an offset gear for driving the secondary shaft, and a clutch arranged between drive shaft and offset gear, have in common the need for a lubricant pump, which entails a correspondingly high level of outlay in terms of construction and an additional energy demand for driving the lubricant pump.
DE 10 2004 056 841 A1 has disclosed a differential-controlled transfer gear with a housing and with an offset gear which comprises a gearwheel set. The output shafts are arranged at an angle with respect to one another. To compensate for the angle, the gearwheel set has level gears. Output shaft sealing rings which seal off a housing interior against oil losses are, as viewed from outside the housing interior, arranged in front of the output shaft bearings with a spacing in each case in the axial direction along the output shafts.